1) The V-MODA M100 is similar to the stock Denon D5000 year 2012, with a bass tilt. However, there is a 5dB drop at 440Hz, a 5 to 8 dB drop between 5kHz and 8kHz, and a bass boost of about 3dB starting around 200Hz and extending to below 20Hz as compared to the D5000. Based on this, you will hear differences between the sonic characters of the M100 and D5000. The M100's recessed midrange at 440Hz is probably most noticeable, but the lower treble drop will be noticeable as well on some music (note that a lot of headphones have this dip, including the Mad Dogs which I've heard and the T50RPs which I own).

2) The Denon AH-D600 is a lot closer to the D5000 year 2012 everywhere in terms of frequency response. There still are dips in the midrange (closer to 250 Hz here) and the lower treble, but they are smaller, and the bass boost relative to the D5000 is less. I've never heard this headphone so I can't say much more about it.

3) A couple headphones you might want to consider are
a) The Yamaha Pro500. It is actually closer to the AH-D5000 than both the M100 and D600 in frequency response, and differs mainly through the lower treble dip. But, there are other issues with this can (low impedance, and some find it uncomfortable). Try before you buy if you can.
b) The Philips Fidelio X1. It is very close to the AH-D5000 in frequency response everywhere except the bass, where the X1 has a midbass hump of about 5dB at 70Hz and rolls off from there, so it misses a little of the lowest end of the D5000. It doesn't have a detachable cord, though, and parts might not be replaceable on it (i.e. long term upkeep might be an issue). I've never heard this headphone. Tyll gave it a positive review.

I own the M100 and Pro500. I like the Pro500 for sound and the M100 for form factor, and wear the M100 outside and the Pro500 inside when I want a bass-tilt sound.

I read through this thread and just wanted to chime in because the D5k and Pro900 are my two favorite headphones and the X1 is rapidly gaining its place among them.

I think the Pro900 has a bit more power in the bass and (with a good source, and amp and proper burn in) comparable treble.

But the Pro900's mid range doesn't stack up.... That was why the D5K was such a good piece to have to compliment the Pro900 (for the average basshead) and because it offered a bit more spaciousness in the sound.

I haven't heard the M100 so couldn't speak to that but the X1 is a nice headphone... kinda like what an open back version of the D5k would be but the open back leaks too much air for it to have that same ultimate authority in the low end that made it so addictive. That aside I think the X1 is one of the best all rounder mid range open back cans on the market.

The JVC DX700 is the closest I have found to matching the D5K.... the build quality is not as aesthetically nice (although it can take more of a beating) but the DX700 gives you everything sonically that the D5K did and then some.... It has the same absolute slam in the low end, has the same energetic treble, and same balanced mid range but the absurdly wide cups on the DX700 give it a super W I D E soundstage.

Problem is that it looks absurd when you wear it.... It is the ultimate headphone geek headphone.... So I will eventually be chasing down another D5K on the secondary market when funds allow.